Method and apparatus for testing the insulating ability of an insulation on an electric conductor

ABSTRACT

A method and an apparatus for touch-free testing of the insulating ability of an electric insulation on an electric conductor, where the testing item ( 8 ), the conductor, continuously is moved between a system of testing electrodes ( 2, 4 ), where the electric current path to the surface of the testing item is established by means of ionized atmospheric air, ion cloud or corona, between the testing electrodes ( 2, 4 ), made by a high frequenced high voltage generator ( 14 ) with low effect, where the testing item ( 8 ) is moved through the center of the electrode system ( 2, 4 ), that defects in the insulation are determined by means of a D.C. voltage established in an electric serial connection with the ion cloud, so that insulation defects in the testing item ( 8 ) will cause an electric current through the electric charge carrier in the form of ions, and that the electric current is registered by a current measuring circuit ( 18 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for testing of the insulatingability and of an insulation on an electric conductor, such as anenameled wire or a cable, and of the type.

2. Description of the Prior Art

U.S. Pat. No. 3,413,541 discloses an apparatus, where the difference ofvoltage for the testing is established between the electric conductorand a mechanical arrangement with rolls; and from U.S. Pat. No.2,488,578 it is furthermore known to use hanging chains to formconnection to the conductor, which insulation has to be tested.Furthermore it is known to transfer the testing voltage to the conductorby means of metal balls or the like, through which the conductor ismoved.

These methods have in common that an insulation defect on the conductor,for instance in the form of a crack or a hole, will be registered as avoltage flashover. The voltage has to be chosen so that it does notexceed the puncture voltage of the insulating material, but is largerthan the puncture voltage in atmospheric air at the same distance as thethickness of the insulating material.

It is furthermore known to use a contact wheel, where the wire to beinsulation tested extends in an angular notch along the periphery of thewheel. That is that puncture may only occur where the sides of the notchtouch the wire. An arrangement, where the wheel is substituted by aconducting media, such as salt water or mercury, which is known from thedescription in Danish patent application No. 2281/88, does not have thisdefect; but causes pollution of the wire to an unacceptable degree,which requires a subsequent cleaning process. The liquid may besubstituted by a metal powder or by metal balls. This will not causepollution of the surface of the wire; but the surface of the wire may bedamaged by grinding.

WO 90/05311 furthermore discloses a test method making use of asuperimposed high A.C. voltage on the testing voltage, whereby apuncture may be obtained where the insulation layer is defective. Thisrequires that the superimposed voltage is adjusted to a voltage betweenthe puncture strength for air, or in this case a gas which ionizeseasily, Argon, and for the measured insulating material. However, inthis case ionizing only occurs if a defect in the insulation occurs.

DD-A-209 912 and DD-A-209 913 discloses a test method, where theconducting media also may be replaced by a sufficient quantity ofelectric charge carriers, ions, which over a suitable axial distance isarranged to surround the wire. Such an electric conducting cloud may bea plasma cloud, or ionized gaz. A plasma cloud generated by a lightedelectric arc is capable of carrying an electric current, by ionizing offor instance a noble gas between the electrodes. An electric arc isaggressive, both towards the electrode materials, the insulatingmaterials and in this connection the item tested, due to the very hightemperature to maintain the arc, and thereby the ionizing. This resultsin unfavourable properties in he form of sporadic damage of the testitem and the insulating material and poor durability of the electrodes.However the test method disclosed in DD-A-209 912 is simplified byobtaining a varying division of the supplied voltage and with it apotential displacement between the light arc and the test voltage bymeans of an ohmic voltage divider.

U.S. Pat. No. 4,151,461 discloses, that the ionizing also may beobtained by continuously discharging of sparks; but to secure continuousionizing it requires high power, 150-800 watts at 100-600 Hz, and ittherefore causes that the electrodes to be quickly worn down.

U.S. Pat. No. 3,263,165 furthermore discloses, that the ionizing ofatmospheric air also may be provided by means of a radioactive isotope,which may establish a current through the atmospheric air. Due toenvironmental reasons is not expedient, and requires intensivemaintenance.

EP-A-0 264 482 discloses the use of laserbeams to generate the chargecarriers, and EP-A-0 367 379 furthermore discloses, that electronemitting materials, such as chemical compounds of barium or calcium mayemit electrons—but only by heating to high temperatures.

The aforementioned six methods have the important advantage, that theinsulating material and the wire are not touched mechanical, which iswhy there is neither wear, tearing or physical damage to the surface ofthe insulation. Hardening of the metal core of the item, which normallymay happen by mechanical bending around rolls or the like, does notoccur. Such hardening is very unfavorable such as, for example, copperwire for spools in electric machines etc.

SUMMARY OF THE INVENTION

The invention is a method for testing the insulating ability of theinsulation on an electric conductor by means of a careful, touch-freemanner by means of simple provisions to establish a sufficient quantityof electric charge carriers in the form of an ion cloud or corona totest quickly and effectively, whether the electric conductor iscompletely covered by a sufficient layer of insulating material, andthereby is capable of insulating for an electric voltage.

The method according to the invention is distinctive in that the ioncloud is established by a high frequency high voltage generator with lowpower, and that defects in the insulation are determined by means of aD.C. voltage established in an electric serial connection with the ioncloud, so that insulation defects in the testing item will cause anelectric current through the electric charge carrier in the form ofions, and at the electric current is registered by a current measuringcircuit. In a careful, touch-free manner and by means of simpleprovisions it hereby becomes possible currently—on-line—to establish asufficient quantity of electric charge carriers in the form of an ioncloud or corona to test quickly and effectively, whether the electricconductor is completely covered by a sufficient layer of insulatingmaterial, and thereby is capable of insulating for electric voltage. Itshould be emphasized that—in order to avoid damaging heat protection ofthe test item—there is made use of high-frequency and high voltage withlow power to establish a permanent corona between the testingelectrodes.

The invention furthermore relates to an apparatus for use with themethod according to the invention, which apparatus comprises a system oftesting electrodes providing a central passage for a tested item, anelectric conductor, so that the tested item may be moved continuouslythrough the passage, a high-frequency high voltage generator with lowpower, being adapted to ionize the atmospheric air in the passage andform a permanent ion cloud or corona, a measuring voltage generator anda current measuring circuit, and wherein the measuring voltage generatorforms a current path through the ion cloud or the corona to the surfaceof the tested item, if insulation defects may occur, as the currentmeasuring circuit also is connected with the electric conductor of thetested item, and wherein the testing electrode system is provided as twosets of electrodes being alternatively placed in a circle around thepassage for the tested item in such a manner, that the electrodesmutually are equally distributed along the circle that is by way ofexample mutually displaced 45°.

The apparatus according to the invention is furthermore with one set ofthe electrodes insulated from the other set of electrodes in such amanner that a permanent ion cloud or corona is formed because of thecapacitive current generated by the voltage from the high voltagegenerator.

In an advantageous manner the insulating material between the set ofelectrodes is of such type (shape and material), that the ionizing doesnot cause puncture, sparks, but continuous discharges in the form of apermanent ion cloud or corona.

In order to optimize the effect of the preferably permanent ion cloud orcorona the electrodes are provided so that only between the respectivecircuits a common current path is formed through the ionized atmosphericair in the form of an ion cloud or corona, so that the current from themeasuring voltage generator may pass through the ion cloud or the coronavia a hole in the insulation of the tested item and further through themetal conductor of the tested item connected with the current measuringcircuit.

Appropriately the apparatus according to the invention is provided sothat one set of electrodes consists of a metal coating on the inner sideof a piece of pipe made from glass or ceramics, which piece of pipe isplaced parallel with the passage for the tested item; while the otherset of electrodes consists of electrode placed perpendicular to thepassage and between first-mentioned set of electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail in the following withreference to the drawings, in which:

FIG. 1 shows a schematic sectional view seen from the side with acoupling diagram for an embodiment for an apparatus to be used by themethod according to the invention, and

FIG. 2 shows a perspective exploded view of an embodiment of anarrangement of sets of electrodes of an apparatus according to theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the coupling diagram shown in FIG. 1 the apparatus of the inventioncomprises a set of electrodes 2 and a set of electrodes 4 eachconsisting of four electrodes arranged in a circle around a centralpassage 6 for a tested item 8, for instance an insulated electricconductor, which insulation coating is continuously tested, while theconductor is moved through the passage with suitable speed. The set ofelectrodes 2 consists of four thin cylindric electrodes being placedperpendicular to the tested item 8 in the passage 6 and mutuallydisplaced 90° along the circle, causing an end part of the electrodes tobe placed close to the central passage 6 of the tested item 8. The setof electrodes 4 consists of metal coatings 10 on the inside of tubepieces 12 made from glass or ceramics, functioning as the insulatingmaterial around the set of electrodes 4 having sufficient electric andmechanic expertise, such as the properties of glass or ceramic. In otherwords FIGS. 1 and 2 show two sets of electrodes with each of the fourelectrodes being mutually displaced 45° from each other.

As it furthermore appears from FIG. 1 there is by means of a highvoltage A.C. generator 14 a high frequency and high voltage and lowerpower which is dependent on the electric properties of the material inquestion; but typically it will be about 60-100 KHz and 3-25 volts. Bymeans of a measuring voltage generator 15, a measuring voltage isimpressed, and by means of a current measuring circuit 18 any faultcurrents caused by holes in the insulation of the testing item 8 aredetected, which tested item—as shown in FIG. 2—similar to the measuringcircuit is connected to the apparatus frame. The measuring signal may bemeasured between the wires 20 and 22.

In other words the object of the invention is to utilize ionizing, whichoccurs by way of example in atmospheric air just before the electricfield intensity in the air is high enough to make a puncture happen.This ionizing is described as corona. To maintain the continuous cloudof ions without a puncture occurring, the electrodes 2 and 4 areseparated by an insulating material which is the tubes 12. The ionizingoccurs continuously by means of a high A.C. voltage with a highfrequency from the A.C. generator 14. The current of the corona iscapacitively coupled with the insulating material 12 functioning as adielectric. Since capacitively coupled current and voltage are phasedisplaced 90° with a capacitive load, the heat generated mainly iscaused only by the dielectric loss in the insulating material 12 of thesystem. For the same reason the power consumption is low, about 5 wattsfor the disclosed system of electrodes. The system of electrodes is inthe embodiment described circumferentially oriented around the testeditem 8, and the embodiment causes that the ionizing of the air to besufficient around the whole diameter of the tested item to perform thetesting. Alternatively, the system of electrodes may be carried out withthe electrodes in line, whereby a plate placed on the insulation, or afoil over a metal roller may be tested.

The function of the apparatus is such that the tested item continuouslyis moved through the central passage 6 of the system of electrodes, andany insulation fault causes an electric current to flow as electriccharge carriers in the form of ions. This is due to the fact that theatmospheric air within the set of electrodes 2 and 4 is madeelectrically conductive in the form of an ionized gas by means of anelectric field intensity causing discharges with high frequency in theform of corona.

The embodiment of the electrodes insures that the ionized atmosphericair contains a sufficient quantity of ions in the center of the systemof electrodes so that a detectable increase of current from themeasuring voltage generator 16 may be detected by the the currentmeasuring circuit 18, if the insulation of the item is defective.

The embodiment insures that there are two circuits, which alone have acommon current path in the ionized atmospheric air, corona. Thereby onlythe current from the measuring voltage generator 16 will be able to runthrough the corona, via a hole in the insulation, and further throughthe conductor of the item, which is connected to the current measuringcircuit 18 via the frame connection to the apparatus. If there are noholes in the insulation of the tested item a current path to the metalcore of the item cannot be established because of the insulatingmaterial.

What is claimed is:
 1. A method of touch-free testing of electricalinsulation of an electrical conductor comprising: continuously moving aninsulated electrical conductor through an array of testing electrodesincluding at least one pair of spaced apart testing electrodes which areseparated by a solid insulator extending along at least one of the atleast one pair of spaced apart testing electrodes and the electricalconductor in a zone of testing which prevents puncture and sparkingbetween the at least one pair of testing electrodes during testing ofthe electrical insulation of the electrical conductor; producing with avoltage generator, which applies a voltage from the voltage generator tothe array of testing electrodes, a continuous corona which contacts themoving electrical conductor; causing an electrical current flow throughany defects in the insulation and the continuous corona; and measuringthe electrical current flow through the defects in the insulation andthe corona.
 2. An apparatus for touch-free testing of electricalinsulation of an insulated electrical conductor comprising: an array oftesting electrodes having a passage through which the insulatedelectrical conductor passes during testing of the electrical insulationwith at least one pair of spaced apart testing electrodes which areseparated by a solid insulator extending along at least one of the atleast one pair of spaced apart testing electrodes and the electricalconductor in a zone of testing which prevents puncture and sparkingbetween the at least one pair of testing electrodes during testing ofthe electrical insulation of the electrical conductor; a coronagenerator, coupled to the array of testing electrodes, which producescorona that continuously contacts the insulated electrical conductorwhen the insulated electrical conductor is located in the passage; avoltage generator which applies a voltage between a reference potentialand the array of testing electrodes; and a current measuring device,coupled to the insulated electrical conductor, which measures currentflow through the insulation and the corona caused by any defect in theinsulation to indicate the presence of the defects.
 3. An apparatus inaccordance with claim 2 wherein the array of electrodes comprises: twosets of testing electrodes, the two sets of testing electrodes beinginsulated from each other by a plurality of solid insulators withindividual testing electrodes of the sets of testing electrodes beingequally displaced from each with the insulated electrical conductorpassing through a central passage therein during testing.
 4. Anapparatus in accordance with claim 3 wherein: the two sets of testingelectrodes are capacitively coupled to each other.
 5. An apparatus inaccordance with claim 4 wherein: each set of testing electrodes areinsulated from each other by the plurality of solid insulators whichextend along the electrical conductor in the zone of testing whichprevents puncture and sparking between the testing electrodes when thecorona generator is activated.
 6. An apparatus in accordance with claim2 wherein: each testing electrode of one set of the testing electrodescomprises a metal coating on an inner side of a cylinder which is thesolid insulator with the cylinder being parallel to the passage and theother set of electrodes are conductors extending outward from thepassage and between the one set of the testing electrodes.
 7. Anapparatus in accordance with claim 6 wherein: the electrode sets aredisposed uniformly circumferentially around the insulated electricalconductor.
 8. An apparatus in accordance with claim 3 wherein: eachtesting electrode of one set of the testing electrodes comprises a metalcoating on an inner side of a cylinder which is the solid insulator withthe cylinder being parallel to the passage and the other set ofelectrodes are conductors extending outward from the passage and betweenthe one set of the testing electrodes.
 9. An apparatus in accordancewith claim 8 wherein: the electrode sets are disposed uniformlycircumferentially around the insulated electrical conductor.
 10. Anapparatus in accordance with claim 4 wherein: each testing electrode ofone set of the testing electrodes comprises a metal coating on an innerside of a cylinder which is the solid insulator with the cylinder beingparallel to the passage and the other set of electrodes are conductorsextending outward from the passage and between the one set of thetesting electrodes.
 11. An apparatus in accordance with claim 10wherein: the electrode sets are disposed uniformly circumferentiallyaround the insulated electrical conductor.
 12. An apparatus inaccordance with claim 5 wherein: each testing electrode of one set ofthe testing electrodes comprises a metal coating on an inner side of acylinder which is the solid insulator with the cylinder being parallelto the passage and the other set of electrodes are conductors extendingoutward from the passage and between the one set of the testingelectrodes.
 13. An apparatus in accordance with claim 12 wherein: theelectrode sets are disposed uniformly circumferentially around theinsulated electrical conductor.
 14. An apparatus in accordance withclaim 6 wherein: each testing electrode of one set of the testingelectrodes comprises a metal coating on an inner side of a cylinderwhich is the solid insulator with the cylinder being parallel to thepassage and the other set of electrodes are conductors extending outwardfrom the passage and between the one set of the testing electrodes. 15.An apparatus in accordance with claim 14 wherein: the electrode sets aredisposed uniformly circumferentially around the insulated electricalconductor.
 16. An apparatus in accordance with claim 7 wherein: eachtesting electrode of one set of the testing electrodes comprises a metalcoating on an inner side of a cylinder which is the solid insulator withthe cylinder being parallel to the passage and the other set ofelectrodes are conductors extending outward from the passage and betweenthe one set of the testing electrodes.
 17. An apparatus in accordancewith claim 16 wherein: the electrode sets are disposed uniformlycircumferentially around the insulated electrical conductor.
 18. Anapparatus in accordance with claim 8 wherein: each testing electrode ofone set of the testing electrodes comprises a metal coating on an innerside of a cylinder which is the solid insulator with the cylinder beingparallel to the passage and the other set of electrodes are conductorsextending outward from the passage and between the one set of thetesting electrodes.
 19. An apparatus in accordance with claim 18wherein: the electrode sets are disposed uniformly circumferentiallyaround the insulated electrical conductor.
 20. An apparatus inaccordance with claim 9 wherein: each testing electrode of one set ofthe testing electrodes comprises a metal coating on an inner side of acylinder which is the solid insulator with the cylinder being parallelto the passage and the other set of electrodes are conductors extendingoutward from the passage and between the one set of the testingelectrodes.
 21. An apparatus in accordance with claim 20 wherein: theelectrode sets are disposed uniformly circumferentially around theinsulated electrical conductor.